1. Field of the Invention
The present invention relates to a camera system including a catadioptric lens, and more particular to a catadioptric lens system suitable for a single lens reflex camer having an automatic focus detection device.
2. Description of the Prior Art
A plurality of kinds of automatic focusing cameras of single a lens reflex type have been proposed. A plurality of types of automatic focus detection devices have been also proposed. The proposed automatic focus detection devices are classified into an active type automatic focus detection device and a passive type automatic focus detection device. The passive type automatic focus detection device is further classified in details into a contrast detection type and a phase difference detection type. The principle of the contrast detection type automatic focus detection device is to detect the sharpness of an image formed by a photographic lens for finding an in-focus condition of the photographic lens. It has a disadvantage that the detectable range of defocus amount is relatively narrow, although it has an advantage that the in-focus condition of the photographic lens can be detected regardless of the F-number thereof. On the other hand, the principle of the phase difference detection type automatic focus detection device is to detect a distance of two secondary images of an image formed by the photographic lens. It has an advantage that the detectable range of defocus amount is relatively wide in comparison with the former contrast detection type automatic focus detection device, although it has an disadvantage that the focus detection thereby is impossible with respect to a photographic lens having a larger F-number beyond a predetermined value. Therefore, the phase difference detection type automatic focus detection device can not be used for detecting a focusing condition of a catadioptric lens having a relatively large F-number.
The reason will be explained in details below. FIG. 1 shows a construction of an optical system used in a conventional phase difference detection type automatic focus detection device. In FIG. 1, an automatic focus detection module AFS, provided in a camera body, comprises a pair of separator lenses 2 for forming two secondary images of an image formed by a photographic lens TL on a charge coupled device (CCD) 4, and the focusing condition of the photographic lens TL with respect to an object located on an optical axis of the photographic lens TL is detected by detecting the distance between the two secondary images. Here, the configuration of light bundles incident on the CCD 4 with respect to an exit pupil plane thereof is shown in FIG. 2. In FIG. 2, EP represents a configuration of the exit pupil of the photographic lens TL, and A.sub.1 and A.sub.2 represent areas through which light rays pass for forming the secondary images on the CCD 4, respectively. The areas A.sub.1 and A.sub.2 are called "incident pupil" of the automatic focus detection module AFS hereinafter.
If the exit pupil EP of the photographic lens TL is small in comparison with the incident pupil A.sub.1 and A.sub.2 of the automatic focus detection module AFS as shown in FIG. 3, the automatic focus detection module AFS can not receives sufficient light through the photographic lens TL for forming the secondary images of sufficient intensity of light. Therefore, the correct detection of the focusing condition of the photographic lens TL may be impossible Therefore, it is necessary to restrict the F-number of the photographic lens to which the focusing condition can be detected by the phase difference detection type automatic focus detection device.
Furthermore, such restriction should be more severe with respect to a catadioptric lens system. Namely, an exit pupil EPr of the catadioptric lens system forms a ring shape as shown in FIG. 4, and therefore a light ray does not pass through the central portion near the optical axis on the exit pupil plane of the catadioptric lens system.